I can't agree strongly enough about the statements already made about 'controlled and contained', etc. Beyond birds and squirrels, cats kill many snakes. A couple of thoughts to add:
1) It isn't just cat poop. Cats can carry several parasites which can infest humans, even though we're not the intended host. Some of these can be, and have been, fatal to humans.
2) Window screens can't be counted on to keep cats, or many other things, in or out.
3) If many cats are poisoned in a neighbourhood, it could be that someone has an open bucket of antifreeze around after changing their engine coolant.
4) Actually, it is not a landowner's prerogative to put out posion or traps (other than live traps) on their property for whatever purpose, or even for the specific purpose of controlling problem animals. One could understand the frustration that might lead them to do it, though.
I'm glad to hear that your sister's cat is getting better, since it is an indoor cat that got loose just this once.